Hexapods Enable Precise 6-Axis Patient Positioning Couches for Radiotherapy

Radiation therapy is becoming a more mainstream way to treat tumors, not only because advances have made it more cost efficient and less intrusive than the classical treatments of surgery and chemotherapy but also because it performs better for many types of tumors.

External beam radiotherapy is mainly applied to tumors located in the head, breast, lung, and prostate. Since the goal is to destroy the tumor cells without harming the healthy tissue around it, positioning the patient precisely relative to the beam is crucial. Based on the average size of human cells, a positioning resolution up to 0.03mm is recommended.

Most current installations are IGRT systems (Image Guided Radiotherapy), using one or several methods to determine the topical patient’s and tumor’s position before and during radiation treatment.

These methods include:

X-ray scans
Optical body surface profiling
Thermal imaging (adding a further dimension to monitor the patient’s position)
Magnetic resonance imaging (MRI)

The aim of these methods is a patient-friendly, frameless stereotactic radiosurgery (SRS), e.g. of brain tumors.

Optical body surface profiling based on visible light

X-ray scans precisely determine the position and the topical position of the tumor

In LINAC based devices, the patient is precisely positioned in relation to the 3D recorded data and radiation source by superimposing previously taken images. A multi-degree of freedom (DOF) operating table (patient couch) plays a key role in the precision of the entire process. Before the radiation treatment can start, the couch needs to move and orient the patient into the correct position in space.

To correctly position the patient to the radiation source, the operating table needs to be moved with six degrees of freedom: Three linear directions (X, Y, Z) and three rotational directions (ΘX, ΘY, ΘZ). Precision better than 0.1mm is required along with a load capacity of 200kg (440lbs) or above.

It is also possible to dynamically adjust the 6-axis couch during the radiation treatment, to counteract unwanted motion. Very high precision and high load capacity of >400lb are required. These requirements, along with freely programmable motion in 6 degrees of freedom, are achieved by PI hexapod 6-axis parallel kinematic positioning systems.

The radio therapy can only begin after the patient and tumor have been positioned correctly to the beam.

Hexapods (derived from Greek for 6 legs) are motion platforms controlled by six actuators (struts). They all act together on one platform and enable movements in six degrees of freedom.

Examples of custom hexapods - rectangular hexapod design and circular hexapod design. For either case, 6 actuators, located between a bottom and top frame, control the three linear (X, Y, Z) and rotary positions (ΘX, ΘY, ΘZ) of the top platform precisely, based on input from the hexapod motion controller. Learn more

Video: 7-Axis motion system consisting of a custom PI Hexapod and PRS200 Rotary stage in a setup with 3D-printed phantoms imaged repeatedly with a cone-beam computer tomography scanner. More information on this research from Johns Hopkins University is available here: https://aiai.jhu.edu/research/#phantom and here: https://jhu.pure.elsevier.com/en/publications/assessment-of-boundary-discrimination-performance-in-a-printed-ph

PI has 3 decades of experience in the design and manufacture of various types of hexapod mechanisms and controllers along with hexapod specific software, including specific designs for patient positioning couches (see drawing below). In addition to almost 100 standard models, a large variety of custom models have been designed, reflecting customer needs with respect to positioning resolution (nanometer to sub-millimeter), speed, dynamics, size, shape, payload (from a few ounces to several tons), and cost.

Hexapods provide many advantages, such as higher stiffness and accuracy, combined with lower mass, and a more compact design compared to conventional ways of creating 6-axis motion systems and are perfectly suited to positioning patients for radiotherapy.

Recent Research at the UK’s Kings College has suggested a robotic system for vitreoretinal surgery based on two H-825 hexapod 6-axis parallel robots.

PI’s C-887 Hexapod Motion Controller – the 5^th generation of in-house designed controllers. PI hexapod controllers provide a large amount of advanced features. All motion commands are easily programmable in Cartesian coordinates, including a virtual, user specific pivot point (center of rotation). Communication is via a number of digital interfaces, including industry standard EtherCat connectivity. Additional analog interfaces can be used for alignment sensor or force sensor feedback, and two more motor outputs can operate auxiliary positioning devices.

Basic design of a low profile, 6-axis hexapod for patient couches

Of course, we can also provide positioning systems with plain linear or rotary motion, or any combinations thereof. Our engineers are ready to solve individual customer requests in radiotherapy. Talk to PI!